Recording system utilizing magnetic deflection

ABSTRACT

An ink jet recording system including means for producing a stream of high speed ferrofluid ink droplets, magnetic deflection means for deflecting said ink stream, said magnetic deflecting means including two spaced pole pieces forming an air gap therebetween located so that said ink stream passes therethrough, said gap being shaped to form a gradient magnetic field therein, said gap further being shaped so that movement of said ink stream in a direction normal to the gradient magnetic field does not result in the ink stream striking a pole piece. Said system is further characterized by a plurality of magnetic deflection means located in the path of said ink stream wherein each deflecting means is capable of applying an increment of deflecting force to said stream.

United States Patent [1 1 Fan et al.

[ RECORDING SYSTEM UTILIZING MAGNETIC DEFLECTION [75] Inventors: George J. Fan, Ossining; Richard A.

Toupin, Briarcliff Manor, both of N.Y.

[73] Assignee: International Business Machines Corporation, Armonk, N.Y.

22 Filed: Aug.30, 1972 21 Appl. No.: 284,822

[52] US. Cl. 346/75, 346/140 [51] Int. Cl. G0ld 15/18 [58] Field of Search 346/75, 1, 140

[56] References Cited UNITED STATES PATENTS 1,882,043 10/1932 Schroter 346/75 X 3,091,762 5/1963 Schwertz 346/74 X 3,287,734 11/1966 Kazan 346/75 X 3,510,878 5/1970 Johnson 346/75 X DIRECTION OF DROPLET DEFLECTION Apr. 16, 1974 Attorney, Agent, or Firm-Roy R. Schlemmer, Jr.

[5 7] ABSTRACT An ink jet recording system including means for producing a stream of high speed ferrofluid ink droplets, magnetic deflection means for deflecting said ink stream, said magnetic deflecting means including two spaced pole pieces forming an air gap therebetween located so that said ink stream passes therethrough, said gap being shaped to form a gradient magnetic field therein, said gap further being shaped so that movement of said ink stream in a direction normal to the gradient magnetic field does not result in the ink stream striking a pole piece. Said system is further characterized by a plurality of magnetic deflection means located in the path of said ink stream wherein each deflecting means is capable of applying an increment of deflecting force to said stream.

10 Claims, 6 Drawing Figures N-UENTEDAPR T 6 I974 $805272 SHEET 1 OF 2 F E6. 1 12\ DEFLECTION 14 DROP 10 iz- PRODUCING Q Q Q Q Q Q q Q GO q c T AN E FRON INK R SDUC R I REsERvOTR 22 TO TNN l RETORN SYNCHRONIZATION DEFLECT'ON CIRCUITRY SIGNAL SOURCE FIG. 28 I do 9 FIG. 2A HHWQT-EE NQE- OTREOTTON OF OROPLET DEFLECTION 52 44 F I 3 BINARY DEFLECTION To INPUT T SIGNAL DEC'MAL 42 v DECODER 40 RESET MG A 40 Q I N l COUNTER 1 1 1 42 CLOCK sTONAF A RECORDING SYSTEM UTILIZING MAGNETIC DEFLECTION BACKGROUND OF THE INVENTION There is an increasing need in the data processing industry for devices to provide permanent visible records.'Two of the more common forms of such records are graphical and alphanumeric data printouts. As will be readily understood, the former relates generally to some sort of a two dimensional graph or plot and the latter generally to printed data of one form or another.

Many types of devices have been used in the past for providing a graphical data output record and these include relatively complex mechanical X-Y recorders wherein electrical motors, for example, drive a moveable stylus to desired coordinates on a sheet of fixed or moving graph paper. Also, various electron beam devices have been utilized to provide various types of graphical records utilizing either photographic recording media, electrochemically sensitive papers, and the like. Most prior art graphical recording systems suffer either from the actual recording media being overly expensive; i.e., special chemically treated papers, photographic films, and the like, or relatively slow; i.e., complex mechanical X-Y recorders having pen styli.

Similarly, many problems have existed in the past particularly in the computer industry in providing alpha-numeric or printed output from computers at a rate compatible with the data processing rates of the more recent highly complex, high speed computing machines. It is well known in the computing art that one of the primary bottlenecks, when printed output is to be provided, is the actual, physical printing of the final documents. Various types of high speed printers have been developed in the past which have been capable of producing relatively high speed outputs, but which are extremely expensive and subject to a multitude of maintenance problems in that the mechanical linkages, etc., are under continuous mechanical stresses which occasion said frequent maintenance and repair.

Mechanical printers also suffer from an additional disadvantage in that their ultimate speed is severely limited by the mechanical inertia of their moving parts. It should be noted that virtually all mechanical printers fall into the category of impact printing wherein the type forming mechanism must come into forceable contact with the record receiving media at some point.

A relatively new printing technology which is applicable, in theory, both to graphical printing as well as alphanumeric printing is that known as ink jet printing.

In such systems, the actual printed characters are formed by moving a stream of ink in a desired fashion, interrupting said stream as necessary and causing said stream to impinge upon a suitable recording surface such as a sheet of moving paper. Such systems are generally referred to as impactless printing and theoretical printing speeds are very high since there is very little mechanical inertia in such systems.

Early issued patents in the ink jet printing area are US. Pat. Nos. 3,596,275 of Richard G..Sweet, and 3,298,030 of A. M. Lewis et al. Said latter patent discloses a number of means by which a character matrix may be generated in the ink jet printing technology.

Both the Sweet and Lewis et al. patents utilize what is known as electrostatic deflection of the ink jet. That is, an electric field is applied to a pair of deflecting plates for moving the charged particles of the ink jet in a desired trace across the recording path.

Electrostatic ink jet recording systems suffer from a number of disadvantages. The charging of the individual ink droplets is quite critical insofar as synchronization of the charging voltage with the formation of the droplets is concerned. The electrostatic deflection effect is somewhat limited in magnitude which results in the deflection system parameters being quite critical. Further, the charge effects between adjacent droplets causes considerable problems.

U.S. Pat. No. 3,510,878 of C. E. Johnson, Jr. discloses a printing system utilizing ink jets with magnetic deflection wherein the particular ink utilized is a magnetic ink or a ferrofluid. Although the Johnson patent discloses magnetic deflection in an ink jet printing system, the specific structure of Johnson has a number of shortcomings which are solved by the present invention as will be set forth subsequently.

US. Pat. No. 3,287,734 of Kazan discloses a magnetic deflection recording system which is the magnetic analog of the Sweet system. In this system it is alleged that the ink droplets are pre-polarized prior to deflection and the direction and degree of polarization determine the amount and direction of deflection. Thus it is stated that each droplet becomes a small bar magnet under the influence of the polarizing field. However, experimentation has indicated that this polarizing effect cannot be reliably obtained due to the magnetic fluids available.

SUMMARY AND OBJECTS OF THE INVENTION It has now been found that an ink jet recording system utilizing the advantages of a ferrofluid ink and electromagnetic deflection thereof can be achieved by utilizing air gap defining pole pieces having a shape such that a pronounced magnetic gradient is set up in said air gap wherein said structure produces minimal possible interference with the ink stream and yet which provides very precise control of the positioning of said stream.

it is accordingly a primary object of the present invention to provide a high speed recording system.

It is a further object to provide such a recording system utilizing impactless recording techniques.

It is a further object to provide such a recording system suitable for the high speed recording of alphanumeric information in a data processing environment.

It is a still further object to provide such a recording system utilizing ink jet techniques incorporating magnetic deflection of the ink stream.

it is a still further object of the invention to provide such a recording system which incorporates an improved magnetic deflection structure having improved control of the recording stream.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an organizational drawing of the present ink jet recording system showing the principle functional components thereof.

FIG. 2A is a detailed drawing showing a single magnetic deflection element illustrating a preferred form of said element.

FIG. 2B is a diagrammatic drawing indicating certain parameters involved in a magnetric deflection system.

FIG. 3 shows the details of the magnetic deflection system comprising a preferred embodiment of the invention incorporating a plurality of separate magnetic deflection stages or elements. 7

FIG. 4A is a top view of a multiple deflection station system, the control electronics for which are shown in FIG. 3.

FIG. 4B is a side detail view of the system of FIG. 4A specifically illustrating the preferred dimensions of the multiple deflection station system shown in FIG. 4A.

DESCRIPTION OF THE DISCLOSED EMBODIMENTS The objects of the present invention are accomplished in general by a method of making a permanent recording representative of an electric information signal comprising the steps of forming a ferrofluid ink stream, directing said stream at a writing surface, generating a gradient magnetic field in the path of said stream, forming said gradient magnetic field so that deflection of said stream into the higher density portions of said field does not result in an impingement of said stream upon the field generating means. According to a preferred form of this invention, the ink stream is a series of droplets wherein adjacent drops may be selectively deflected.

According to an additional feature of the invention said gradient magnetic field is generated between a pair of spaced pole pieces such that a wedge-shaped gap is formed therebetween but wherein said gap is open at 1 both sides thereof. Thus, excessive deflection of said stream into the denser field areas in the portions of said gap, where said pole pieces are in closest proximity, results in said stream merely passing out through the narrower portion of said gap rather than striking either of said pole pieces. According to a still further aspect of the invention, a plurality of individual magnetic deflection means are utilized wherein the length of said gap in the direction of travel of said ink stream is approximately the diameter of individual droplets making up said stream and wherein the spacing of adjacent deflection stations is approximately the spacing between such drops.

By providing the aforementioned gap structure, any tendency of the ink stream to strike and thus collect on and foul the pole pieces forming said gap is avoided. Maintenance problems occasioned by such pole piece fouling are essentially eliminated.

Utilization of plural magnetic deflection stations with suitable signal switching means provided therefor substantially increases the fineness of control of the deflection of adjacent drops since the deflecting field may be continued over a substantial portion of the path of travel of individualdroplets and yet only a desired deflection signal will be affecting a particular droplet. Further, an adjacent droplet may have a considerably different deflection signal without substantial interference or interaction of the deflecting signals upon subsequent drops. However, this mechanism will be set forth and described more fully subsequently.

It should first be understood that a ferrofluid ink as described herein for use with the disclosed ink jet recording system comprises a fluid having magnetic polarizabilty, or in other words, one which maintains essentially fluid physical characteristics but which also will behave under the influence of a magnetic field much the same as any other soft or paramagnetic material. A satisfactory ferrofluid to be utilized'as an ink in the present system would be one which would have substantially uniform magnetic properties so that its action is obviously predictable. It should be understood, however, that the type of fluid suitable for magnetic inks used in the present invention is unlike more or less well known magnetic clutch fluids of the type consisting of ferrite particles which chain together and essentially solidify under a strong magnetic field. A ferrofluid consists of a colloidal suspension of submicronsized ferrite particles in a carrier fluid or vehicle such as kerosene, water, or silicone base fluids. A dispersing agent is added to prevent floculation. This type of ferrofluid is described by R. E. Rosenweig, Magnetic Fluids, International Science and Technology, July 1966, pgs. 48-56. A further reference is Some Applithe sense of any sort of permanent or semipermanent' magnetic alignment within the individual ferromagnetic materials within a drop, as such an effect would be merely a random and temporary condition but it is the overall gross ferromagnetic property of the ink which allows the gradient field to pull the ferromagnetic droplet into the more concentrated portion of to FIG. 1, there is shown an overall diagramatic organiza-tion of a typical ink jet recording system constructed in accordance with the present invention. The organization is quite similar to those utilized in other known ink jet systems. It comprises a nozzle 10 which produces the ink jet stream as a series of droplets as will be described fully subsequently. The source of the droplets comprises the transducer arrangement 12 which receives ink from an ink reservoir, not shown. The operation of such drop producing transducers is well known in the art, reference being specifically made to the Sweet US. Pat. No. 3,596,275 wherein a typical drop producing transducer is illustrated.

The stream of droplets 11 passes through the electromagnetic deflecting means 16 and impinges upon the record surface shown at 14. A deflection signal source 20 provides a deflecting signal to the magnetic deflecting means which in turn produces a gradient magnetic field within the path of the ink stream 11. It will be noted especially in referring to FIG. 2 that the direction of the gradient field is fixed by the physical construction; however, the strength may vary in accordance with the strength of the deflection signal applied to the winding coils. It will also be noted that a synchronization means 18 is shown which performs the necessary function of providing synchronization between the application of the deflecting signal to the deflecting means 16 and the ink droplet forming mechanism whereby proper deflection signals will be applied to desired ink droplets. A shoot or gutter 22 is located downstream from the deflection means and generally in the path of a maximally or minimally deflected droplet to intercept same and return droplets so intercepted to the ink reservoir. This means is provided, as will be well understood, when it is desired that certain droplets should not reach the recording surface. This is due to the fact that it is much easier to provide for interception of the droplets in the jet stream than to physically turn the nozzle off periodically. This is in contrast to the Johnson U.S. Pat. No. 3,5l0,878 referenced previously wherein a low speed droplet binary mechanism is utilized; i.e., turning a valve on and off. The presently disclosed system is a free jet which obtains the fastest possible droplet formation for high quality recording.

By utilizing the particular gap geometry of which a preferred embodiment is shown in FIG. 2A, it is possible to obtain maximum deflection of the stream of droplets including even deflection all the way through the air gap and out the other side without having to worry about droplets impinging upon the pole pieces. This can be an important advantage during startup and shutdown. Referring now to FIG. 2A, it will be noted that there is shown amagnetic structure 30 having a coil 32 surrounding same for generating the requisite -magnetic flux'in the armature structure 30. It will be further noted that the air gap 34 is essentially wedgeshaped and is open at both sides. As is apparent from the density pattern of the magnetic flux shown in the figure, in the narrower portion where the reluctance or length of the air gap is less, the magnetic flux tend to be more concentrated. This gradient field or variation in magnetic flux density across the air gap causes the magnetic ink droplets to be deflected in toward the region of the air gap wherein said flux is more dense in much the same way as with a simple solenoid which pulls a moveable ferro-magnetic slug into the densest region of the coil structure. It will be evident that with the configuration shown in FIG. 2A, with the narrow side of the air gap being open, in the event that an extremely strong signal is applied to the energizing coil 32 which would be sufficient to deflect the ink droplet past the inner boundary of the air gap due to inertial effects and the like, there is little possibility of the droplet striking one of the pole pieces. As stated previously, the advantages of this structure are obvious insofar as freedom of ink fouling of the pole pieces is concerned. It will further be readily understood that slight changes in the shape of the pole pieces may be made without departing from the spirit and scope of the invention. For example, the surface of the pole pieces could be slightly cupped rather than straight lines as shown in the figure.

Referring now to FIG. 2B, the principles of the magnetic deflection of such an ink jet system will be specifically set forth. The concept is essentially quite simple and involves passing the stream of magnetic fluid droplets close to one of the pole pieces of an electromagnetic deflecting element placed perpendicular to the undeflected stream. As described previously, it is necessary that a gradient in the field be established whereby the ferromagnetic ink droplet is pulled into the denser region thereof. The following is a theoretical explanation of the physical attraction of the droplet by such a magnetic field. As shown in the FIG. and as used in the subsequent description, A the distance between droplets, 6 angle of deflection, v velocity, d spacing of undeflected droplet from pole piece.

For small angles of deflection 0, we have where tip is the impulse (change of momentum) delivered to a droplet passing near a pole of an electromag-' net, and p is its initial momentum. Thus,

where is the mass density of the fluid, V its volume,

and v its velocity.

The magnetic force on a saturated droplet is given by F lo sat (N is a unit vector in the direction of H) It is possible to carry out the calculations so as to determine a precise value of F for every location of the droplet relative to a pole face. Here we content ourselves with a crude estimate of the total impulse in a direction normal to v delivered by a single wedge-shaped gap magnetic transducer. This is given approximately by:

(m is the mass of the droplet; L'is the width of the pole face) For small angles of deflection we have M ma /m Choosing the following typical values:

M gauss 10 cycles/sec.

p 1.1 gm/cc A 10' cm This is the maximum angular deflection to be expected from a single deflecting station. By reducing the estimate for the distance of closest approach, or by the use of more than one deflecting magnet, it is possible to increase A0. Of course, diminishing the velocity of the jet increases the angular deflection since A varies inversely as the square of the jet velocity. This degrades the rate of printing only linearly.

All of the above description and analysis of the magnetic deflection phenomena is believed to be essentially accurate; however, it should be clearly understood that there may be factors involved in the deflection which are not presently known.

In the prior description of the deflection effect it should be noted that deflection of the droplet prior to saturation thereof is proportional to the product of the field H and the field gradient 8H. After the droplet is saturated, it is proportional only to the gradient.

As stated previously, the shape of the air gap or wedge can be changed to a somewhat different contour to maximize the I-I'6I-I product.

Another way to obtain maximum deflection is to use a static uniform magnetic field superimposed on a variable deflecting field. The strength of this field should be sufficient to polarize to saturation or near saturation each magnetic fluid droplet. In this case, formula (3) above applies with H replaced by the variable deflecting field and all the subsequent formulas and estimates are unaffected. The advantages of superimposing the static field on the variable field are: l) the deflection is more linear with input current; and 2) there is more deflection in certain current ranges; i.e., the variable field need not be used to obtain satu ration of the droplets.

While a single deflecting element will obviously provide deflection in such a magnetic deflection ink jet recording system, the use of a multiple array of deflecting transducers or elements has particular promise. This concept is illustrated in FIG. 3 wherein a plurality of transducers 40 are shown, each having the general configuration, for example, of the transducer shown in FIG. 2A.

The multiple transducer embodiment exemplified by FIG. 3, allows ready implementation ofdigital input information for the deflection control signal. Referring to the FIG., it will be noted that the multiple transducers are representedby the reference numeral 40. In this system, it is desired to utilize each transducer for a unit of deflection. Thus, for example, if there were transducers and each transducer were given the same deflection signal, a total of 10 units of deflection of a given ink droplet would occur providing all 10 transducers were energized. If, however, it were only desired to give three units, only three of the transducers would be energized, etc. With the embodiment of FIG. 3 a convenient way of obtaining this sort of deflection control is typified. The AND circuits 42 and the Binary to Decimal Decoder 44 perform the function of selecting the particular transducers to be energized while the Ring Counter 46 under control of a clock signal steps a deflection signal along the group of transducers at substantially the same speed as the ink droplet travels. Thus, if four units of deflection are desired, the first four transducers would be activated by energizing the outputs of the first four lines of the decoder 44. Assuming a suitable reset signal is applied to the Ring Counter, as clock signals are received, the first four transducers would be energized sequentially. As stated previously, the speed of propagation of the deflection signal along the group at AND circuits 42 must, of necessity, be equal to the physical speed or velocity of the droplets passing through the deflecting transducer element air gaps. It is believed to be obvious to provide compensating controls to maintain synchronization between droplet speed or velocity and the actual signal propagation rate through the transducer array due to ambient effects.

It is believed that the various components making up the deflection system as shown in FIG. 3 would be well known to a person skilled in the art. Reference is hereby made to the publication Pulse, Digital and Switching Waveforms by Millman and Taub, McGraw-Hill Book Co., 1965, which specifically discloses various types of ring counters in the section beginning on page 693 and a number of different types of decoders are discussed beginning on page 349.

With this arrangement each deflecting element at any given time has a signal which only affects the droplet within its immediate physical range or area of attraction and thus there is very little interference between the deflection of adjacent droplets.

It will, of course, be readily appreciated that other control circuits could readily be designed for controlling the multiple deflecting elements by those skilled in the art. For example, a fairly straightforward shift register could be used with adjacent stages connected to adjacent deflection elements. In this case a variable magnitude deflection signal could be applied to one end of the shift register which would in effect follow a particular droplet along the deflection path. Still other control means could readily be adapted for use with the overall recording system concept of the present invention.

FIG. 4A is a top view of a multiple deflecting station embodiment wherein the individual deflecting elements 16A, 16B and 16C are shown displaced in the direction of deflection of the droplet stream. Four discrete positions of deflection of the droplet stream 11 are illustrated in this figure. The one is for a 0 deflection. The next shows a deflection of 0 The next shows a deflection of 0 plus 6 and finally the fourth or leftmost stream indicates a deflection of 0 0 6 These deflections may be obtained with an analog deflecting signal applied to each of the windings 40 as shown in FIG. 3 under control of the disclosed pulse generation circuitry whereby a somewhat more linear deflecting effect is obtained by displacing the stations along the arcuate deflection path, However, with the embodiment disclosed in FIG. 4A, a somewhat different deflecting philosophy can also be used. For a deflection of 0 obviously none of the stations would be energized. For a deflection of 0 the first deflecting station 16A would be energized. For a deflection of 0 both 16A and 168 would be energized and for a full deflection of 0, 0 0 all three stations would be successively energized as a desired droplet passed therebetween. The design of the pulse circuitry for producing this function would be quite straightforward as for the embodiment of FIG. 3. Thus, assuming a particular droplet is to have two increments of deflection as it passed station 16A, a pulse must appear on 16A, after the pulse leaves the pulse no longer is present and as the same droplet passes station 16B, 168 they are fed with a similar deflection signal. Thus, each of the deflecting stations is supplied with a simple straightforward stream of binary bits which are effective to energize or de-energize same. The synchronization of the pulse streams as well as thecharacter encoding are believed to be quite straightforward, and obvious to those skilled in the art.

Experience has shown that an optimum dimension for each of the elements in such a system is that the width of a deflecting element or transducer should be approximately equal to the drop diameter and spacing of the actual elements must, of necessity, be approximately equal to the spacing of the droplets comprising the stream (or a multiple thereof) for the deflecting system to operate. By using a plurality of such deflecting elements, smaller currents may be utilized with each of the deflecting elements to obtain a given amount of deflection because with a single deflecting station a very large signal must be utilized to obtain a maximum desired deflection of the droplet.

FIG. 4B clearly indicates the optimal dimensioning of a multiple deflection station system such as described in the previous paragraph. It may be seen that each of the deflecting magnet pole pieces 16A, 16B, and 16C have a width substantially equal to the diameter of an individual ink droplet making up the stream 11. Similarly, the spacing of the individual deflecting elements is equal to the spacing of the individual droplets making up the stream.

It would also be possible to enhance the maximum deflection available with such a multiple deflection transducer array by contouring or spacing the location of the individual deflecting elements along the path of deflection of the droplets.

An operating embodiment of the present invention has been built and successfully tested utilizing the concepts illustrated in the embodiment of FIG. 1. Excellent drop selecting capabilities as well as deflection and character forming characteristics were demonstrated.

The ferrofluid ink utilized consisted of a colloidal water base suspension of magnetite particles having an average diameter of 100A. The viscosity of the ink at room temperature was between 6 and 10 centipoise at low shear rate. The surface tension of the ink was 35 dynes/cm and its saturation magnetization was emu/gm. Its density was 1.2 gm/cm. The ink was supplied to a nozzle 1.4 mil inside diameter at a pressure of psi. The velocity of the droplets was approximately 6 meters/sec. A piezoelectric crystal transducer was connected to the nozzle structure to introduce pressure waves in the ink and thus produce a droplet frequency of 20 kilohertz. The droplet size was approximately 2.3 times the nozzle diameter or 3.4 mils. The spacing between successive droplets was approximately 12 mils.

The deflecting magnet was ferrite and had substantially the configuration shown in FIG. 2A. The pole pieces formed a wedge shaped gap, as illustrated, wherein the angle between a plane normal to the axis of the pole pieces and the face of the pole piece was 15 (a total angle of between the pole faces). The gap between the two adjacent pointed portions of the pole pieces (narrowest point) was 7 mils. The deflecting magnet was 6 mils wide in the dimension parallel to the axis of the gap and 60 mils wide in the dimension perpendicular to the aixs and the gap. The strength of the gradient of the magnetic field for a signal producing maximum desired deflection of the droplet stream was greater than 10 Gauss/cm.

The deflecting magnet was spaced approximately 0.3

cm from the nozzle and 2.5 cm from the recording medium. Deflection of the droplet stream on the recording medium in the described embodiment was zero for no signal applied and approximately 1 mm. with a maximum signal used producing the magnetic field above described. The energizing windings for the magnet consisted of 50 turns of No. 32 AWG wire fed with a signal of SOOmilliamps for maximum deflection.

Excellent character production has been achieved utilizing the above described apparatus as a part of overall deflection systems together with very crude character generating circuitry and record transport apparatus. No fouling of the pole pieces occurred, even on start up, before the electronic circuitry stabilized. The advantages of the present invention have been clearly proven in the above-described example and show great promise for electromagnetic deflection in ink jet printing systems of the future.

In summation, the advantages of the present magnetic deflection system, especially over conventional electrostatic systems, are that with an electromagnetic system, no high voltages are required for deflecting as with an electrostatically charged droplet stream. No charging or polarizing operation whatsoever is required, since it is not possible to permanently magnetically polarize such droplets. This latter fact makes synchronization problems somewhat less critical with this type of a magnetic deflection system than is the case with electrostatic systems. The use of magnetic ink in the printing system also-results in a ferromagnetic record. If certain types of printing formats are utilized, subsequent readout of the printed record would be possible which, in certain applications, would have many advantages.

While the invention has been disclosed and described with reference to certain preferred embodiments thereof, it will be readily be appreciated that certain modifications and changes could be made by persons skilled in the art without departing from the spirit and scope of the present invention.

What is claimed is:

1. An ink jet recording system including means for producing a stream of high speed ferro fluid ink droplets, electromagnetic deflection means for deflecting said ink stream, said magnetic deflecting means including two spaced pole pieces defining an air gap and an associated winding for energizing same, said gap being shaped to produce a gradient magnetic field thereacross, said ink jet stream being located with respect to said pole pieces so that with no deflecting signal applied to said winding said stream occupies a space substantially equi-distant from each pole piece and wherein energizing said winding causes deflection of said stream in a plane perpendicular to said gradient magnetic field, and said air gap being further shaped so that movement of said ink stream in said deflection plane in response to said gradient magnetic field cannot result in said stream striking such pole pieces.

2. A recording system as set forth in claim 1 including a plurality of such electromagnetic deflecting means located in the path of said ink stream such that energization of each of said deflecting means causes incremental deflections of said stream.

3. A recording system as set forth in claim 2 wherein said pole pieces define a wedge-shaped gap therebetween wherein said gap is open at both sides and wherein the higher density portion of said gradient magnetic field Occurs adjacent the two portions of said pole pieces which are closest together and wherein the ink streams will be deflected into said denser field region.

4. A recording system as set forth in claim 2 wherein each of said deflecting means includes an electrical winding wherein the passage of current through said winding produces said magnetic field in said air gap and circuit means for selectively energizing desired ones of said plural deflecting means to cause predetermined deflection of individual ink droplets defining said stream.

5. A recording system as set forth in claim 4 wherein said circuit means includes means for energizing a particular deflecting means with a particular deflecting signal only while a particular droplet is within the air gap of said deflecting means. 6. A recording system as set forth in claim 4 wherein said successive deflecting means are located in an arcuate path downstream of said ink jet producing means whereby each deflecting means is in an optimal location to produce a deflection effect upon a given magnetic droplet which has been incrementally deflected by the immediately preceding deflecting means.

7. A recording system as set forth in claim 6 wherein said circuit means includes N AND gates, each having an output connected to energize one of N magnetic deflecting means, a binary to digital decoder which accepts a binary input signal and energizes from 1 through N output lines depending upon the binary input, each of said output lines being connected to one i of said N AND circuits, a ring counter having N stages,

each stage having an output connected to one of said N AND circuits, a clock signal source for stepping said counter and a reset means for resetting said counter at the end of a particular deflection cycle.

8. A recording system as set forth in claim 7 wherein the width of each electromagnetic deflecting means has a width approximately equal to the diameter of a droplet of ink making up said ink stream and wherein the magnetic field thereacross, each said deflecting means being energized by an electrical winding, said ink jet stream being so located with respect to said pole pieces that with no deflecting signal applied to said winding said stream occupies a space substantially equi-distant from each pole piece and wherein energizing said winding causes deflection of said stream in a plane perpendicular to said stream in a plane perpendicular to said gradient magnetic field, said air gap being further shaped so that movement of said ink stream in said deflection plane in response to said gradient magnetic field cannot result in said stream striking such pole pieces,

switching circuit means for controlling the energization of said plural deflecting means, said circuit means including timing circuit means for energizing a particular deflecting means only as long as a particular ink droplet is within the magnetic field produced by that deflecting means, and

droplet. 

1. An ink jet recording system including means for producing a stream of high speed ferro fluid ink droplets, electromagnetic deflection means for deflecting said ink stream, said magnetic deflecting means including two spaced pole pieces defining an air gap and an associated winding for energizing same, said gap being shaped to produce a gradient magnetic field thereacross, said ink jet stream being located with respect to said pole pieces so that with no deflecting signal applied to said winding said stream occupies a space substantially equi-distant from each pole piece and wherein energizing said winding causes deflection of said stream in a plane perpendicular to said gradient magnetic field, and said air gap being further shaped so that movement of said ink stream in said deflection plane in response to said gradient magnetic field cannot result in said stream striking such pole pieces.
 2. A recording system as set forth in claim 1 including a plurality of such electromagnetic deflecting means located in the path of said ink stream such that energization of each of said deflecting means causes incremental deflections of said stream.
 3. A recording system as set forth in claim 2 wherein said pole pieces define a wedge-shaped gap therebetween wherein said gap is open at both sides and wherein the higher density portion of said gradient magnetic field occurs adjacent the two portions of said pole pieces which are closest together and wherein the ink streams will be deflected into said denser field region.
 4. A recording system as set forth in claim 2 wherein each of said deflecting means includes an electrical winding wherein the passage of current through said winding produces said magnetic field in said air gap and circuit means for selectively energizing desired ones of said plural deflecting means to cause predetermined deflection of individual ink droplets defining said stream.
 5. A recording system as set forth in claim 4 wherein said circuit means includes means for eNergizing a particular deflecting means with a particular deflecting signal only while a particular droplet is within the air gap of said deflecting means.
 6. A recording system as set forth in claim 4 wherein said successive deflecting means are located in an arcuate path downstream of said ink jet producing means whereby each deflecting means is in an optimal location to produce a deflection effect upon a given magnetic droplet which has been incrementally deflected by the immediately preceding deflecting means.
 7. A recording system as set forth in claim 6 wherein said circuit means includes N AND gates, each having an output connected to energize one of N magnetic deflecting means, a binary to digital decoder which accepts a binary input signal and energizes from 1 through N output lines depending upon the binary input, each of said output lines being connected to one of said N AND circuits, a ring counter having N stages, each stage having an output connected to one of said N AND circuits, a clock signal source for stepping said counter and a reset means for resetting said counter at the end of a particular deflection cycle.
 8. A recording system as set forth in claim 7 wherein the width of each electromagnetic deflecting means has a width approximately equal to the diameter of a droplet of ink making up said ink stream and wherein the spacing of said heads is approximately equal to the spacing between individual ink droplets or a multiple thereof.
 9. An ink jet recording system including means for producing a stream of high speed ferro fluid ink droplets, a plurality of electromagnetic deflection means for deflecting said ink stream, each said magnetic deflecting means including two spaced pole pieces defining an air gap, said gap being shaped to produce a gradient magnetic field thereacross, each said deflecting means being energized by an electrical winding, said ink jet stream being so located with respect to said pole pieces that with no deflecting signal applied to said winding said stream occupies a space substantially equi-distant from each pole piece and wherein energizing said winding causes deflection of said stream in a plane perpendicular to said stream in a plane perpendicular to said gradient magnetic field, said air gap being further shaped so that movement of said ink stream in said deflection plane in response to said gradient magnetic field cannot result in said stream striking such pole pieces, switching circuit means for controlling the energization of said plural deflecting means, said circuit means including timing circuit means for energizing a particular deflecting means only as long as a particular ink droplet is within the magnetic field produced by that deflecting means, and means for subjecting predetermined droplets to a total deflecting force of a predetermined magnitude.
 10. An ink jet recording system as set forth in claim 9 wherein said circuit means includes means for supplying deflection signals of a fixed predetermined magnitude to the plural deflection means whereby the magnitude of deflection of a particular droplet is determined by the number of deflection means energized during the traverse of the overall deflection system by such droplet. 